erofs: get correct count for unmapped range in fscache mode

For unmapped range, the returned map.m_llen is zero, and thus the
calculated count is unexpected zero.

Prior to the refactoring introduced by commit 1ae9470c ("erofs:
clean up .read_folio() and .readahead() in fscache mode"), only the
readahead routine suffers from this. With the refactoring of making
.read_folio() and .readahead() calling one common routine, both
read_folio and readahead have this issue now.

Fix this by calculating count separately in unmapped condition.

Fixes: c665b394 ("erofs: implement fscache-based data readahead")
Fixes: 1ae9470c ("erofs: clean up .read_folio() and .readahead() in fscache mode")
Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20221104054028.52208-3-jefflexu@linux.alibaba.comSigned-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/fscache.c

@@ -296,15 +296,16 @@ static int erofs_fscache_data_read(struct address_space *mapping,
 		return PAGE_SIZE;
 	}
 
-	count = min_t(size_t, map.m_llen - (pos - map.m_la), len);
-	DBG_BUGON(!count || count % PAGE_SIZE);
-
 	if (!(map.m_flags & EROFS_MAP_MAPPED)) {
+		count = len;
 		iov_iter_xarray(&iter, READ, &mapping->i_pages, pos, count);
 		iov_iter_zero(count, &iter);
 		return count;
 	}
 
+	count = min_t(size_t, map.m_llen - (pos - map.m_la), len);
+	DBG_BUGON(!count || count % PAGE_SIZE);
+
 	mdev = (struct erofs_map_dev) {
 		.m_deviceid = map.m_deviceid,
 		.m_pa = map.m_pa,